Don't we all love these high and soft passages? They make us look so good all the time! Just kidding of course!

I'm afraid there is no trick that will guaranty a 100% success rate but there are a few things that can help you.

First, you need to plan your dynamics according to your artistic judgement rather than your physical abilities. If you just try to play as soft as you can, you'll always try to get softer and softer and your playing will become uncomfortable. This will cause you to miss notes and create more stress. You should always play as soft as you SHOULD instead. This way, you will get to a nice soft sound and keep it there unless the conductor asks for less. They don't always do by the way contrary to what most people say! By having a "bottom" to your dynamics, you give yourself a realistic goal and take some stress off your shoulders.

Second, take a good breath before playing. When we see piano written in our part, we very often have the reflex to take a small breath. It's much easier to play a soft passage with full lungs.

Third, let go a nice piano sound rather then restraining your forte.

These tricks can help you playing soft in the high range but there is no guaranty. You need to be patient with this aspect of your playing. Avoid over-practicing a passage before a concert. You'll eventually miss a note if you try to play it 100 times in a row which will make you more nervous. Concentrate on what you have to do to make it work and trust yourself.